Particulate fractions prepared from meadow hay, ranging in size from 0·1 to 2 mm, were incubated with rumen digesta from four cannulated Romney sheep fed the same hay. The rates of gas production, lipolysis of corn oil and hydrogenation of linoleic acid were measured.
The rate of gas production per g fermentable particles (FP) was approximately 30% lower with 1–2 mm than with the 0·1–0·4 mm particles. However, per m2 surface area the rate for the larger particles was found to be approximately 600% greater.
The rates of lipolysis of triacylglycerols and hydrogenation of linoleic acid were respectively 25 and 60% higher per g FP and 1100 and 1200% higher per m2 FP surface area with the 1–2 mm particulate fraction.
The same hay particulate fractions were incubated with pure cultures of Ruminococcus flavefaciens, since this organism is active in both lipid metabolism and cellulose fermentation. The rate of gas production and the number of organisms adhering to the particles were determined.
The effects of particle size on gas production were similar to those found when incubations were carried out with rumen digesta. Per g FP the rate was 40% lower with 1–2 mm than with 0·1–0·4 mm particles. However, per m2 surface area the rate was found to be approximately 450% greater with the former.
It was further found that although the density of the bacterial population on 1–2 mm particles was 600% higher than on the 0·1–0·4 mm particles, the rate of gas production per 109 bacteria remained unchanged.
We conclude that per m2 surface area fermentation, lipolysis and hydrogenation were more rapid with particles ranging from 1 to 2 than from 0·1 to 0·4 mm in size. This was due, at least in part, to microbial population density.